Many systems determine distances to an object based on an optical time-of-flight (TOF) measurements. More specifically, in such systems, which can be referred to as TOF based distance measurement systems, a pulse of light is emitted by a light source, reflected from a distant object, and received by a light detector. Electronics associated with the light source and the light detector measure the elapsed time from the time the pulse was emitted to the time the reflected pulse (or portion thereof) was received, and the elapsed time is used to calculate a distance at which the object is located. In such systems, the measured length of time (i.e., time delay) that it takes a pulse of light to travel roundtrip corresponds to the distance to the object, at approximately 6.6 nanoseconds/meter. More specifically, since light travels at approximately 3.3 nanoseconds per meter (i.e., the speed of light is approximately 3.3 nanoseconds per meter), the distance to an object can be determined using the equation d=(c*t)/2, where c is the speed of light and t is the time delay.
If the goal is to measure this time delay of a few nanoseconds to accuracies of 1 centimeter or less, then a precision of approximately 66 picoseconds would be required. However, such precision is difficult to achieve for various reasons. For example, there are delays in the signal path that an electrical pulse travels before it reaches the light source (typically a laser) and causes the light source to emit a corresponding pulse of light. Similarly, there are delays in the signal path from the light detector to electronics that determine TOF measurements. These added delays are typically on the order of several nanoseconds, which are comparable to the delay being measured. Making such precision even more challenging is the fact that these delays in the signal paths can vary significantly over time with changes in temperature, e.g. by several nanoseconds, significantly reducing the precision of the measurements.
Conventional approaches to compensating for the variable delays in the signal paths include re-calibrating a TOF based distance measurement system periodically by positioning a reference target at a precisely known distance, determining offsets at specific times, and correcting TOF readings based on such determined offsets. Such an approach is reasonable, but has several problems. First, a reference target needs to be built into the system, which typically requires additional hardware resources, complications and costs. Further, there is a need to periodically perform measurements associated with the reference target, and potentially as part of every TOF reading. This will slow down the system by the amount of the extra readings required for the dynamic recalibrations. Additionally, there is a need to correct each TOF measurement taken, using the reference calibration measurements, which will take additional time and possibly additional hardware to perform this dynamic calibration.